1. Field of the Invention
The present invention relates to a light-emitting element driving circuit, and in particular, to a light-emitting element driving circuit which changes brightness of a light-emitting element.
2. Description of the Related Art
In recent years, there are more occasions where a video image such as one-segment broadcasting is viewed on a main liquid crystal display of a portable phone, and a light-emitting element driving circuit which changes the brightness of the backlight of the liquid crystal is sometimes equipped in the portable phone. The light-emitting element driving circuit applies an automatic adjustment of the brightness of the backlight of the liquid crystal corresponding to the ambient luminosity, in order to optimize the visibility. More specifically, the light is adjusted such that the brightness of the backlight is increased to achieve brighter display when the ambient environment is bright and the brightness of the backlight is reduced to achieve darker display when the ambient environment is dark, so that the visibility is optimized. JP 2005-310854 A discloses a driving circuit which applies brightness adjustment of a light-emitting element.
In addition, a main liquid crystal screen is provided on a portable phone, and when a key button or the like of the portable phone is operated when the main liquid crystal screen is viewed, the backlight is set in the high brightness (normal mode) and a bright screen is achieved. When a state of no operation is continued, the main liquid crystal screen is automatically set to the low brightness (slight emission mode) and a dark screen is realized, to inhibit the current consumption. JP 2005-310854 A discloses a drive circuit which applies brightness adjustment of a light-emitting element.
Moreover, as described above, because there are more occasions where an video image such as one-segment broadcasting is viewed on the main liquid crystal display of the portable phone, a light-emitting element driving circuit which changes brightness of the backlight of the liquid crystal is sometimes equipped in the portable phone. In the light-emitting element driving circuit, a control to change the brightness of the backlight of the liquid crystal according to content of the video image signal is applied in order to display a clear image with a superior contrast. More specifically, the light is adjusted such that a bright image is displayed brighter with the brightness of the backlight increased and a dark image is displayed darker with the brightness of the backlight reduced. JP 2005-310854 A discloses a driving circuit which applies brightness adjustment of the light-emitting element.
However, when the current value of current flowing through the backlight is increased in order to increase the brightness of the backlight corresponding to the ambient environment becoming brighter, a forward voltage (Vf)) of the backlight is increased, and the terminal voltage of the cathode terminal of an LED forming the backlight is reduced. When the terminal voltage of the cathode terminal is reduced to a value less than a set voltage, a voltage boosting circuit is activated to boost the terminal voltage of the anode terminal. However, there may be cases where, after the voltage is once boosted, for example, the ambient environment becomes darker, the current value of the current flowing in the backlight is reduced in order to reduce the brightness of the backlight, and the forward voltage (Vf)) of the backlight is reduced. In such a case, although the terminal voltage of the cathode terminal of the LED is increased, the voltage boosting circuit may still be in an operating state, and consequently, a wasteful current consumption is caused.
Moreover, during the normal mode, the brightness is high. When the current value of the current flowing in the backlight is increased in order to increase the brightness of the backlight, the forward voltage (Vf)) of the backlight is increased, and the terminal voltage of the cathode terminal of the LED forming the backlight is reduced. Similar to the above, when the terminal voltage of the cathode terminal is reduced to a value less than the set voltage, the voltage boosting circuit is activated and the terminal voltage of the anode terminal is boosted. However, there may be cases where, after the voltage is once boosted, for example, the device is set in the slight emission mode (low brightness), the current value of the current flowing in the backlight is reduced in order to reduce the brightness of the backlight, and the forward voltage (Vf)) of the backlight is reduced. In such cases, if the voltage boosting circuit is in the operating state even after the terminal voltage of the cathode terminal of the LED is increased, wasteful current consumption is caused.
Furthermore, when the brightness is increased for a bright image, the current value of the current flowing in the backlight is increased in order to increase the brightness of the backlight, the forward voltage (Vf) of the backlight is increased, and, consequently, the terminal voltage of the cathode terminal of the LED forming the backlight is reduced. Again, when the terminal voltage of the cathode terminal is reduced to a value less than the set voltage, the voltage boosting circuit is activated to boost the terminal voltage of the anode terminal. However, there may be cases where, after the voltage is once boosted, for example, the brightness is reduced for a dark image, the current value of the current flowing in the backlight is reduced in order to reduce the brightness of the backlight, and the forward voltage (Vf) of the backlight is reduced. In such cases, if the voltage boosting circuit is in the operating state even after the terminal voltage of the cathode terminal of the LED is increased, wasteful current consumption is caused.